Director fluctuations and anisotropic reorientation of a liquid crystal: A deuterium NMR study

The spectral densities of motion were determined by deuteron NMR relaxation measurements in the nematic and smectic-A and -C phases of 4-n-pentyloxybenzylidene-${\mathrm{d}}_1$-4’-heptylaniline and 4-n-pentyloxybenzylidene-4’-heptylaniline-2,3,5,6-${\mathrm{d}}_4$ at two different Larmor frequencies. These spectral densities can only be interpreted by invoking director fluctuations and molecular reorientation as relaxation mechanisms for the deuteron spins at the methine and ring sites. We compared the model of Nordio and co-workers [Nordio and Busolin, J. Chem. Phys. 55, 5485 (1971)] and the third-rate model of molecular reorientation [Vold and Vold, J. Chem. Phys. 88, 1443 (1988)] and found that the third-rate model is more appropriate in the present study. Rotational diffusion constants were determined as a function of temperature in the mesophases of 4-n-pentyloxybenzylidene-4’-heptylaniline.